[Rd] 0/1 vector for indexing leads to funny behaviour (PR#8389) (maybe a documentation deficiency?)

[Prof Brian Ripley]

?"[" says

See Also:

      'list', 'array', 'matrix'.

      '[.data.frame' and '[.factor' for the behaviour when applied to
      data.frame and factors.

      'Syntax' for operator precedence, and the _R Language_ reference
      manual about indexing details.

and the `indexing details' are indeed where it says they are.

This is not an introductory topic, and it makes sense to have the details 
in only one place and refer to it.  That help page is already over-loaded.


On Tue, 13 Dec 2005, Tony Plate wrote:

> Yes, 0/1 (numeric) are intended to be used as index vectors -- and they
> have the semantics of numeric indices, which is that 0 elements in the
> index are omitted from the result.  This can be a very useful mode of
> operation in many situations.
>
> I was going to write "This is described in both the introduction to R,
> and in the documentation for '['", except that I checked before I wrote
> and was surprised to be unable to any discussion of zeros in indexing in
> any of the first three places I looked:
>
> (1) help page for '[' (There is discussion of zero indices here, but
> only in the context of using matrices to index matrices, not in the
> context of ordinary vector indices).
>
> (2) Section 2.7 "Index vectors: selecting and modifying subsets of a
> data set" in "An Introduction to R", which does say this about numeric
> indices:
>     2. A vector of positive integral quantities. In
>        this case the values in the index vector must
>        lie in the set {1, 2, . . . , length(x)}
> (This seems to commit the sin of not telling the whole truth.)

No. Zero is not a positive integer.

> (3) Section 5.5 "Array Indexing.  Subsections of an array" (In "An
> Introduction to R")
>
> Question for others: did I miss something obvious, or is this a
> documentation deficiency that zeros in indices are not discussed in 3 of
> some obvious first places to look?
>
> If indeed this is a documentation deficiency, I'm happy to contribute
> documentation patch, but I await other opinions before spending any time
> on that.
>
> -- Tony Plate
>
> rasche at molgen.mpg.de wrote:
>> Full_Name: Axel Rasche
>> Version: 2.2.0
>> OS: Linux
>> Submission from: (NULL) (141.14.21.81)
>>
>>
>> Dear Debuggers,
>>
>> This is not a serious problem. Are 0/1 vectors intended to be used as index
>> vectors? If yes, there is a bug. If not, it leads just to some funny behaviour
>> rather than an error message.
>>
>> In the appendix is some simple code to reproduce the problem. A logical vector
>> as.logic(a) helps by indexing the vector b. The 0/1 vector a just returns the
>> first value "a". But as many times as there is a 1 in a.
>>
>> Best regards,
>> Axel
>>
>>
>> Appendix:
>>
>> b = c("a","b","c","d")
>> a = c(0,1,1,0)
>> b[as.logical(a)]
>> b[a]
>> a = c(1,0,1,0)
>> b[as.logical(a)]
>> b[a]
>> a = c(0,1,1,1)
>> b[as.logical(a)]
>> b[a]
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>
> ______________________________________________
> R-devel at r-project.org mailing list
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>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595